Electronic control module and method for controlling the operation and safety of at least one radiant tube burner

ABSTRACT

The invention relates to a control module for controlling at least one radiant tube burner, the burner comprising a fuel supply valve, an oxidant supply valve and a combustion fume discharge conduit, wherein the control module comprises: a means for measuring the quality of combustion, installed in the combustion fume discharge conduit of said at least one burner, a unit for measuring the fuel flow rate, a unit for measuring the oxidant flow rate, and a means for driving said at least one burner, acting on the opening percentages of the oxidant and fuel supply valves of said at least one burner in order to adjust the ratio of the oxidant flow rate to the fuel flow rate on the basis of the information delivered by the means for determining combustion quality.

The invention concerns an electronic control module and a method for optimally controlling the combustion and safety of industrial radiant tube burners equipping horizontal or vertical lines for the continuous heat treatment of metal strips.

Referring to FIG. 1 of the drawings, it is shown a schematic example of a part of a vertical line for the continuous heat treatment of metal strips according to the state of the art. It comprises a zone Z1 for preheating the strip 2, for example by hot gas jets, a zone Z2 for heating the strip by radiant tube burners, a zone Z3 for maintaining the temperature of the strip also equipped with radiant tube burners and zones Z4 and following undetailed used for other processing of the strip, for example its cooling.

This line is composed of an insulated enclosure 1 in which the strip 2 enters on a variety of rollers 3 guiding it in a multitude of vertical passes. At the top of each vertical pass are arranged radiant tubes 4, 5 schematically represented by rectangles. On a modern annealing line, the number of radiant tubes—thus burners—installed can be between 200 and 400. Each of these burners is controlled individually and works in on-off mode, with the radiant tubes operating for example in push-pull mode.

In this FIG. 1, the radiant tubes 4 whose burners turned off are shown in white, the radiant tubes 5 whose burners turned on are shown in black.

The maximum heating power P_(max) that can be produced by the heating zone Z2 corresponds to the simultaneous switching on of all the radiant tubes.

According to the state of the art, when the heating power P_(required) required for zone Z2 is less than P_(max), for example equal to 60% of P_(max), each of the burners of the radiant tubes in the heating zone is ignited for 60% of the cycle time, typically set at 1 or 2 minutes.

It is understood that the heating power P_(required) required at any moment is obtained by adjusting the operating time of each radiant tube burner during a portion of the cycle time equal to the percentage of P_(required)/P_(max)

FIG. 2 is a column of radiant tubes 4, 5 located on one side of the strip 2, which runs on rollers 3. The supplies/exhaust of each of the radiant tubes 4, 5 in this column are shown in this figure; the supply of oxidant, for example the combustion air, is schematized by 6, the supply of liquid or gaseous fuel, for example natural gas, by 7 and the exhaust of flue gas by 8. It is understood that according to the location of the fluid connections of the radiant tubes on the supply/exhaust ducts, the air supply or gas supply or flue gas exhaust pressures are different for each of the radiant tubes.

Subsequently, to simplify the description of the invention, we will use the term “gas” to designate the fuel supplying the burners, whatever its nature, and the term “air” to designate the oxidant, regardless of its oxygen content.

The changes in the operating speed of the furnace according to production, of the strip format or of the temperature it must reach, lead to a change in the number of burners in operation, which induces air, gas or flue gas pressure fluctuations on the burners.

On the other hand, the ignition and extinction of a burner can also induce pressure fluctuations on the burner ducts located nearby or in the same zone of the furnace.

In addition, increasing the temperature of the radiant tube reduces the excess air.

It can also be seen at the production sites where the heat treatment lines are installed that the nature or the composition of the gas can fluctuate, sometimes in significant proportions, for example with variations in heating value of +/−10% compared to an average value. The equipment can also be supplied with several types of gas, for example natural gas and coke oven gas having very different characteristics in density or heat values. It can be seen that the operating conditions of the burners can be extremely variable according to air or gas supply or flue gas exhausting pressures, the characteristics of the gas used, the operating temperatures of the radiant tube, the influence of the ignition and extinction of neighboring burners in the column, in the zone or in a fluid circuit.

These variations can be fast and add up to the speed of the ignition and extinction cycle of the burners in on-off mode, for example with 30 or 60 ignition and extinction cycles per hour, which can cause significant disturbances in the operation of the burner alone and in all the burners of a column, the zone or a fluid circuit.

All these variations can cause significant fluctuations of the air-gas ratio compared to the desired theoretical value and/or significant production of pollutants such as CO (carbon monoxide), or NOx (nitrogen oxides).

To solve this problem, it is customary to choose excess air values of +10% to +20% with respect to the stoichiometric value so that the combustion is carried out whatever the conditions of air and gas supply and the characteristics of the gas. This additional part of combustion air does not participate in the combustion. On the contrary, the energy required to heat it is lost energy at the expense of the installation's efficiency.

It can be seen that the equipment according to the state of the art does not make it possible to effectively control the quality of the flame according to the variations in the characteristics of the gas or its supply, and during the flame ignition and extinction phases, which occur in large numbers on an installation comprising a large quantity of burners operating in on-off mode.

It will be understood that the air-gas ratio at each moment of the burner ignition, operation and extinction phases is the result, at each moment, of the opening percentage of the air and gas valves, of the air and gas pressure in supply ducts and the actual heating value of the gas with respect to the theoretical setting value. These differences are particularly important during the flame ignition and extinction phases according to the opening and closing characteristics of the valves, their actual sealing, the wear of their sealing devices and the variations in the characteristics of the air and gas supplies (e.g. pressure variations due to clogging of pipes) or heat values variations in the gas.

These transient phases of approximate control over the quality of the combustion, that is to say the air and gas flow rates and/or the ratio between the air and gas flow rates (the air/gas ratio) have a set of consequences:

-   -   Increased production of pollutants, particularly nitrogen oxides         (NOx) whose release rates are increasingly limited, and         sometimes even taxed,     -   The possibility of producing unburnt byproducts (unburned fuel),         in particular CO, which reduces the efficiency of the         installation and creates a risk of explosion in the ducts,         plenum and final flue gas exhaust circuit that could thus         jeopardize the safety of equipment and people,     -   The production of an uncontrolled combustion atmosphere that may         be oxidizing or reductive, which could reduce the service life         of the equipment exposed to this atmosphere, for example radiant         tubes and generally, all equipment made from refractory steel         operating at a high temperature and exposed to the combustion         gases of the burner(s).

The current control equipment, installed according to the standards in force, for example EN 746-2 and EN 298, control the existence of the flame without the quality of said flame being verified. This means they are open-loop operating modes that do not optimize the combustion.

It can be seen that all the imperfections of the state of the art lead to defects in the control of the combustion of each of the burners at all times, affecting different aspects of the installation, in particular the reduction of combustion efficiency, degradation of the pollutant levels emitted, resistance of the materials in contact with the combustion gases, creation of risks of gas explosion in the ducts, plenum and final flue gas exhaust circuit or more generally risks to equipment or personnel located nearby.

These combustion control defects concern rapid phenomena, on the scale of the burners' operating cycle or the opening/closing times of the gas and/or air supply valves. These short times of poor control over the combustion and the safety of the burner's operation require the implementation of rapid control devices, preferably local, as close to the controlled burner as possible, and operating in the form of closed control loops with fast reaction times.

Some commercial equipment includes sensors located in the air and gas supplies or in the flue gas to make corrections to the operating conditions of the burners, but none makes it possible to optimize each operating phase of the on-off cycle and to compensate for variations in heating value of the supply gas.

In addition, equipment according to the state of the art does not make it possible to quickly detect a malfunction in a radiant tube, whether it is the failure of an element or a degraded mode of operation.

The electronic-control module and the method according to the invention make it possible to optimize the combustion of the radiant tube burners, to reduce the quantities of pollutants emitted, to compensate for the variations in the heating power of the supply gas, to improve combustion efficiency by reducing the excess air required to ensure the proper operation of the burner and quickly detect a malfunction on said radiant tube.

According to the invention, a control module for at least one radiant tube burner, the burner comprising a fuel supply valve, an oxidant supply valve and a combustion flue gas exhaust duct, the control module is characterized in that it comprises:

-   -   a means of measuring the quality of the combustion installed in         the combustion flue gas exhaust duct of at least one burner,     -   a fuel flow measuring device,     -   an oxidant flow measuring device,     -   a means for controlling at least one burner, acting on the         opening percentages of the oxidant and fuel supply valves of at         least one burner to adjust the oxidant flow rate/fuel flow rate         according to the information delivered by the combustion quality         control device.

Advantageously, the control module comprises a means for calculating the combustion power Va of the fuel using the data provided by the combustion quality measuring device, the fuel flow measuring device and the oxidant flow measurement device, the calculated Va value by means of calculation being compared with a theoretical value to detect a deviation exceeding a predefined threshold.

The means for controlling the quality of the combustion could be a residual oxygen sensor.

Even more advantageously, the module may be able to control two radiant tube burners, the combustion power Va of the fuel being calculated for each burner, based on the data supplied by the oxidant flow and fuel flow measuring devices and the information delivered by the combustion quality control device, the two Va values obtained for the two burners being compared in order to detect a deviation exceeding a defined threshold.

The invention also includes a method for controlling at least one radiant tube burner, the burner comprising a fuel supply valve, an oxidant supply valve and a combustion flue gas exhaust duct, the method being characterized in that it consists of:

-   -   control the operation of at least one burner with a regulation         of the opening percentage of the oxidant and fuel supply valves         of at least one burner in a desired oxidant/fuel ratio from the         information delivered by a device measuring the quality of         combustion installed in the combustion flue gas extraction duct         of at least one burner,     -   calculate a value of the combustion power Va of the fuel         supplying at least one burner, in particular from the data         provided by oxidant and fuel flow measurement devices and the         information provided by the device for measuring the combustion         quality of at least one burner, and comparing this combustion         power value Va with a theoretical value to detect a deviation         exceeding a defined threshold.

Advantageously, the control method also makes it possible to:

-   -   control two radiant tube burners,     -   calculate a fuel combustion power value Va for each burner, in         particular from the data supplied by the oxidant and fuel flow         measuring devices and the information delivered by the device         for measuring the quality of the combustion of said burner, and         comparing the two Va values obtained for the two burners to         detect a difference exceeding a defined threshold.

The invention thus provides a fast and efficient system for managing the operation of radiant tube burners installed in large numbers in an industrial furnace. It optimizes combustion and reduces the amount of pollutants produced while ensuring the safe operation of the burners. The invention provides a solution to controlling the burners even when the supply gas has variable characteristics (heat value or supply fuel pressure), by controlling the amount of gas depending on the amount of air to permanently maintain the air/gas ratio required for each burner.

In the following the invention is explained in detail based on an example embodiment with reference to FIGS. 2 and 3 of the pictures.

-   -   FIG. 2 is a partial schematic representation in elevation of the         fluid distribution ducts to the burners according to the state         of the art,     -   FIG. 3 is a schematic representation of a radiant tube assembly         according to an example embodiment of the invention.

As shown in FIGS. 2 and 3 of the drawings, the radiant tube burners are supplied by air 6 and gas 7 ducts. The air supply of the burner is equipped with a flow measuring device, for example a diaphragm 13 and a differential pressure sensor 12 and an electrically or pneumatically controlled opening valve 14, possibly with an opening position feedback signal.

The combustion air is heated by the flue gases in a heat exchanger schematized by 10 to supply the burner 20 with hot air.

The gas supply of the burner comprises a flow measuring device, for example a diaphragm 16 and a differential pressure sensor 15 and two electrically or pneumatically controlled opening valves 17 and 18 performing the function of dual sealing according to EN 746-2 and possibly at least one delivers a feedback of open position (in the figure valve 18) and a pressure switch 26 between valves 17 and 18. The burner 20 is thus supplied with gas and air.

The controlled opening valves 14, 17 and 18 may also be equipped with sensors or limit switches to confirm the position of the valves at full opening or closing.

The burner 20 is equipped with a flame detection device 21, for example an ultraviolet-type optical cell, and an ignition device 22, for example an ignition electrode.

The radiant tube is equipped with temperature sensors, for example at least one thermocouple 25 for measuring its surface temperature and a sensor 24 located on the wet flue gas exhaust 8 of the radiant tube 4 to control the quality of combustion, for example a residual oxygen sensor.

The combustion system is equipped with an electronic control module 23 located near the burner, with output 23 a and input 23 b signals. The input signals according to the example presented are the positions of the controlled valves 14 and 18, flame detection 21, the air and gas flow measurements 12 and 15, the residual oxygen in the wet flue gas measured by the sensor 24 and the temperature of the tube measured by the thermocouple 25. The output signals are the controls of the valves 14, 17 and 18 as well as the ignition control 22.

Finally, a digital link makes it possible to transmit and receive information between a centralized control/command system and the electronic control modules 23 and/or between the electronic control modules 23.

This electronic control module provides all the functions processed by the burner control systems existing on the market according to the state of the art and as defined in the standards, in particular the functions of ignition operation sequencing, burner extinction and safety related to each of these operating phases. It also has a combustion control and a failure control as detailed below.

In on-off mode, the proposed system regulates a quantity of air, which is the reflection of the instantaneous power to be delivered by the burner in its various operating phases, using the valve 14. The differential pressure sensor 12 is connected to the electronic control module 23, which calculates the instantaneous flow of air delivered to the burner.

The sensor 24 for measuring residual oxygen in the flue gas is connected to the electronic control module 23, which determines the quantity of gas necessary to meet the required oxygen level in the wet flue gas in the different operating phases of the burner such as ignition, stabilized operation and extinction and it regulates this flow by controlling the valve 18.

In the same way on the gas circuit, the differential pressure sensor 15 is connected to the electronics module 23, which calculates the instantaneous flow rate of gas delivered to the burner.

The air and gas flow rates are calculated using the formula described in ISO 5167-2 which integrates the geometric characteristics of the primary measuring elements 13, 16, the parameters relating to the properties of the fluids and operating conditions such as atmospheric pressure, pressures, temperatures and densities which are common dynamic data or individual measured data and transmitted to the electronic control module 23.

From the combustion parameters Va (stoichiometric air, or the combustion power of a fuel gas corresponding to the amount of air necessary and sufficient to ensure the complete combustion of the gas volume unit), the required air factor (or aeration rate) noted n, and the air/gas ratio noted R (R=n×Va) and other parameters specific to each of the fluids, the electronic control module 23 calculates the rate of residual oxygen expected in the wet flue gas and adjusts the gas flow to maintain the required oxygen level in the wet flue gas. The invention thus maintains the quality of the combustion whatever the gas and air supply pressure fluctuations.

The invention also proposes other functionalities such as a gas valve tightness test sequencer controlled by the pressure switch 26 as well as a protection in case of exceeding a maximum operating temperature controlled by the thermocouple 25.

The sensor 25 for measuring the temperature of the radiant tube 4 is connected to the electronic control module 23. The module 23 can thus control the shutdown of the burner in the event the radiant tube exceeds a maximum safety temperature.

We understand the interest of this invention for the end user of the furnace because, in addition to improvements in the production process, it reduces the cost of fuel consumed by the facility and lowers taxes that may be demanded based on quantities of pollutants emitted.

According to another essential characteristic of the invention, the electronic control module 23 makes it possible to rapidly detect a malfunction in the radiant tube to which it is connected and to place said tube in the safety position if the malfunction is deemed to be serious.

Depending on the nature of the signal received indicating a malfunction, in particular depending on the element concerned, the electronic module will issue an alert locally and/or to the centralized control/command system, while keeping the radiant tube concerned in service, or stopping it by placing it in the safety position.

As we have seen previously, the electronic control module 23 exchanges information with the devices for measuring and controlling air 12, 13, 14 and gas flow rates 15, 17, 18, 26, as well as the residual oxygen sensor 24 and the temperature sensor 25. The electronic control module 23 can thus detect a discrepancy between the information provided by one of these elements or sensors, the information provided by the other elements or sensors, and the theoretically expected data.

For example, this discrepancy may consist of:

-   -   A discrepancy between the flow rate measured on the air or the         gas and the opening of the control valve regulating said flow         rate,     -   A discrepancy between the measurement of the residual oxygen         content and that expected with respect to the measurements of         air and gas flow rates,     -   A discrepancy between the measured temperature of the radiant         tube and that expected.

From the data provided by the devices measuring the oxidant and fuel flow rates and a sensor measuring the content of residual oxygen in the combustion flue gas of the burner, the electronic control module 23 calculates the Va of the fuel and compares it to the theoretical Va of the fuel. This theoretical Va is advantageously supplied to the module by the centralized control/command system. It can also be directly input in the module by an operator. In the event of a discrepancy between the value of the calculated Va and the theoretical value of the fuel, the electronic control module 23 emits an alert beyond a certain differential threshold. When this difference reaches a second higher threshold, the radiant tube is stopped and secured. The first threshold is for example 10% deviation and the second is 15% deviation.

The theoretical value of Va of the fuel is for example calculated according to the composition of the gas according to the following formula in which the chemical formulations of the gases are to be replaced by the content of these gases in the fuel expressed in m³ of gas per m³ of fuel:

Va=H₂×2.36+CO×2.38+CH4×9.54+C₂H₄×14.4+C₂H₆×16.84+C₃H₆×21.84+C₃H₈×24.37+C₄H₈×29.64+C₄H₁₀×32.41+C₅H₁₂×40.87−O₂×4.77

Advantageously according to the invention, the electronics module 23 is placed in the immediate vicinity of the radiant tube that it controls. This allows a fast exchange of information between the electronic control module and the elements placed on the radiant tube due to a reduced cable length. This solution makes it possible to control and secure radiant tube assemblies faster than they would be through a centralized control/command system. The proximity between the electronic control module and the radiant tube to which it is connected also facilitates the intervention of the operators during the commissioning of the equipment and during its maintenance.

Advantageously according to the invention, the electronic control module 23 is connected to two radiant tubes located close to each other. It can detect a different behavior of the two radiant tubes that may reveal a malfunction of one of the two.

This solution is particularly advantageous because it makes it possible to erase the disturbances that would be related to a variation in the characteristics of the gas and/or air, these being common to the two radiant tubes.

For example, if the electronic control module 23 detects a deviation between the residual oxygen content announced by the sensor 24 of one of the radiant tubes and the flow rates measured in the air and the gas of this radiant tube, this deviation can be interpreted as being linked to a change in the composition of the fuel and its Va. In this situation, the control module 23 according to the invention verifies if the same deviation is present on the second radiant tube. If this is the case, it is a change in the characteristics of one of the fluids. If this is not the case, it is a malfunction of a component on the first radiant tube and an alert is given by the electronic control module. This analysis also makes it possible to detect the perforation of a radiant tube as it would result in the furnace atmosphere entering the tube, if the radiant tube operates in push-pull mode (the pressure inside the tube is lower than that outside the tube), and therefore a drop in the residual oxygen content measured in the flue gas.

As we have seen, the electronic control module of the invention comprises, in an optimized embodiment:

-   -   state-of-the-art safety functions and the imposition of existing         standards,     -   control over the operation of one or two burners with         closed-loop regulation of the openings of the air and gas supply         valves, these loops operating in an air/gas ratio which depends         on the value of the oxygen desired in the wet flue gas at all         points in the operating cycle (ignition, stabilized operation,         extinction), whether the burners are operating in on-off mode or         proportional mode,     -   consideration of data on air and gas characteristics, in         particular the compositions, temperatures, supply pressures and         the heating value of the fuel, for controlling the air-to-gas         ratio of the burner operation,     -   the control of the air-gas ratio of the operation of the burner         carried out in closed loop from the measurement of the residual         O₂ in the flue gas validated by the flow calculations,     -   a system for calculating the Va of the gas to verify that it is         in conformity with that provided by the centralized         control/command system and to trigger an alert if there is a         deviation beyond a defined threshold,     -   control of the combined opening and closing phases of the air         and gas valves performed in real time at each point of this         opening or closing sequence to maintain the desired air/gas         ratio,     -   the periodic verification of the tightness of the gas valves         with safety of the burner if the test is not validated. 

1. A control module for at least one radiant tube burner, the burner comprising a fuel supply valve, an oxidant supply valve, a combustion flue gas exhaust duct, a fuel flow measuring device, and an oxidant flow measuring device, a combustion quality measuring device installed in said combustion flue gas exhaust duct in at least one burner, the control module comprising: a means for controlling said at least one burner, acting on opening percentages of the oxidant and the fuel supply valves of the at least one burner to adapt a ratio of an oxidant flow rate/a fuel flow rate depending on information delivered by the combustion quality measuring device. a means for calculating fuel combustion power Va using data provided by the combustion quality measuring device, the fuel flow measuring device and the oxidant flow measuring device, the calculated fuel combustion power Va value being compared with a theoretical value in order to detect a deviation exceeding a predefined threshold.
 2. The control module according to claim 1, wherein the combustion quality measuring device comprises a residual oxygen sensor.
 3. The control module according to claim 1, wherein the control module is configured to control two radiant tube burners, the combustion power Va of the fuel being calculated for each burner from the data supplied by the oxidant and fuel flow measuring devices and the information delivered by the combustion quality control device, the values of Va obtained for the two burners being compared in order to detect a deviation exceeding a defined threshold.
 4. A radiant tube burner comprising a fuel supply valve, an oxidant supply valve, a flue gas exhaust duct, a fuel flow measuring device, and an oxidant flow measuring device, a combustion quality measuring device installed in a combustion flue of said at least one burner, and a control module according to claim
 1. 5. A method for controlling at least one radiant tube burner, the burner comprising a fuel supply valve, an oxidant supply valve and a flue gas exhaust duct, the method comprising: controlling operation of at least one burner with a regulation of an opening percentage of the oxidant and fuel supply valves of said at least one burner according to a desired oxidant/fuel ratio from information delivered by a combustion quality measuring device installed in a combustion flue gas exhaust duct of said at least one burner, calculating a combustion power value Va of fuel supplying at least one burner, from data supplied by oxidant and fuel flow measuring devices and the information delivered by the combustion quality measuring device of at least one burner, and comparing this combustion power value Va with a theoretical value in order to detect a deviation exceeding a defined threshold.
 6. The method according to claim 5, further comprising: controlling two radiant tube burners, calculating a fuel combustion power value Va for each burner, from data provided by the oxidant and fuel flow measuring devices and information delivered by the combustion quality measuring device of said burner, and comparing the Va values obtained for the two burners in order to detect a deviation exceeding a defined threshold. 